The last glaciation of Aran Island and Cruit Island, County Donegal, north-west Ireland

THE LAST GLACIATION OF ARAN ISLAND AND CRUIT ISLAND,

CO. DONEGAL, NORTH-WEST IRELAND

JASPER KNIGHT

(Received 30 July 2012. Accepted 27 September 2012.)

Abstract

This paper describes geomorphological and sedimentary evidence from Aran Island and

Cruit Island (County Donegal, north-west Ireland), informed by evidence from across west

Donegal, which helps to show the processes of ice advance and retreat and subglacial

conditions during the last (late Midlandian) glaciation. Extensive abraded rock surfaces

show clear evidence for ice overtopping of the islands. Erosional landforms include striae,

flutes developed in bedrock, whalebacks, roches mountonnees and meltwater channels.Depositional landforms include erratic boulders and a single marginal moraine on Aran

Island. Glacial or glacially-influenced sediment deposition that does not have a geomorphic

expression took place into bedrock hollows during an ice advance stage, probably around

the time of the last glacial maximum. The dominance of erosional signatures likely reflects

the stripping by ice of a surficial cover of weathered pre-glacial granite debris. The general

absence of subglacial and ice-marginal sediments across the region likely reflects the granite

substrate and coarse granitic weathering products, which did not favour development of a

thick deformable sediment pile.

Introduction

Several recent studies have highlighted the dynamic

behaviour of the last (late Midlandian) ice masses in

County Donegal, north-west Ireland, in particular

with respect to the timing of advance and retreat

phases and changes in the position of ice dispersal

centres (McCabe and Clark 2003; Ballantyne et al.

2007; Clark et al. 2009; Smith and Knight 2011).

Fewer studies have been concerned with the geo-

morphological and sedimentary evidence for glacial

processes during these phases (Knight and McCabe

1997; Knight 2009, 2011). This is in part due to the

relative paucity of depositional evidence in lowland

areas, even though drumlins, moraines and other

features have been previously mapped within moun-

tain valleys (Charlesworth 1924; Dury 1958, 1964;

Colhoun 1972) and within the larger embayments

(Wright 1912; Knight and McCabe 1997). Glacial

evidence from the rocky islands of west-coast

Donegal (which from north to south include Gola,

Owey Island, Cruit Island, Aran Island, Rutland

Island and Inishfree Upper) has been poorly studied

in comparison to the mainland. However, this

archipelago of islands is an important potential

source of evidence for reconstructing late-glacial�Holocene sea-level changes, because of the high

preservation potential of intertidal organics within

rocky embayments (Shaw and Carter 1994; Brooks

et al. 2008). These rocky embayments can also

preserve glacigenic, glacially-moved and/or Qua-

ternary slope sediments formed in association with

late Midlandian ice. Examples of such sediments are

discussed in this paper.

Large-scale models of British�Irish ice-sheet

evolution during the last glaciation all show that

Irish ice, including from Donegal mountain sources,

extended onto the western Atlantic shelf, thus

Irish Journal of Earth Sciences 30 (2012), 49�58

doi: 10.3318/IJES.2012.30.49

# 2012 Royal Irish Academy

49

inundating the west Donegal islands (Clark et al.

2004; Fretwell et al. 2008) (Fig. 1). This fits with

geophysical evidence for landforms interpreted as

terminal moraines of likely last glacial maximum

(LGM) age on the Atlantic shelf (Benetti et al. 2010;

O Cofaigh et al. 2012). A more detailed ice sheet

model based on 10Be ages from exposed rock

surfaces suggests LGM-age maximum ice thick-

nesses of around 700m over the Donegal mountains,

and ice thickness at the west coast of around 500m

(Ballantyne et al. 2007) (Fig. 1). Together, this

evidence shows that the west Donegal islands

were completely covered by ice during the late

Midlandian. Despite this, little evidence for glacial

processes has been presented previously from these

islands.

This paper describes geomorphological and

sedimentary evidence from the two largest west

Donegal islands (Aran Island [Arainn Mhor] and

Cruit Island) (Fig. 2). A central aim is to provide

information on the processes and patterns of late

Midlandian glaciation in extreme west Donegal,

about which very little evidence is known despite the

long-acknowledged role of this glaciation in shaping

the Donegal landscape. In detail, the paper

(1) describes the geological setting and previous

work in the region; (2) presents evidence for glacial

landforms and sediments from Aran Island and

Cruit Island; and (3) interprets this evidence with

respect to the processes and patterns of late Mid-

landian ice advance and retreat in west Donegal.

Geological setting and summary of previous work

The west coast and islands of County Donegal are

underlain mainly by the Thorr granite (including

granodiorite) into which The Rosses granite was

emplaced (Stevenson et al. 2008). Both are Late

Silurian/Early Devonian in age. The majority of

Aran Island, however, is underlain by the Ards

Quartzite Formation which stratigraphically lies

within the lower part of the Precambrian Dalradian

Supergroup (Long and McConnell 1999). Grano-

diorite and granite from the Thorr granite outcrop

on the south-eastern and southern fringes of Aran

Island. Dolerite dikes aligned WNW-ESE are

locally present within all rock types in the region.

Exposed granite surfaces are present throughout the

region. These exposed surfaces are significant

because rock surface weathering during the

Tertiary�Quaternary was likely an important me-

chanism of loose sediment generation, and may

have exerted a strong influence on the dynamics of

overlying ice throughout the Quaternary, through its

control on substrate properties such as capacity for

subglacial water storage, discussed later. It is

notable that Cruit Island is relatively flat (highest

100 km

IrishSea

Malin Sea

AtlanticOcean

BBA B

BloodyForeland

Land above 200 m a.s.l.

Cruit Island

8°30′W

55°0

0′N

54°4

0′N

8°00′W

North Atlantic Ocean

TheRosses

Owey Island

Aran Island

Rutland Island

Inishfree Upper

Gola

Fig. 1*Map of (A) County Donegal showing the locations of Aran Island and Cruit Island (see Fig. 2) and other places named in thetext. Land over 200m asl is shaded. (B) Late Midlandian ice surface contours (m asl) and ice flow vectors are marked after Ballantyneet al. (2007). Note the ice margin terminates offshore (no ice surface data).

50 Irish Journal of Earth Sciences (2012)

point 32m asl) whereas Aran Island has much

greater relief (highest point 227m asl).

Bedrock surfaces throughout west Donegal are

glacially scoured and have a litter of large erratic

boulders, especially across The Rosses where they

have been used to reconstruct ice dispersal patterns

(Charlesworth 1924) (Fig. 1). Based on interpreted

patterns of striae cross-cutting (Smith and Knight

2011), westerly ice flow in Donegal was followed by

north-westerly ice flow as a result of a southward

shift in late Midlandian ice centres. The effects of

bedrock scouring are clearly evident across the

region, with whaleback summits, roches mounton-

nees and striae commonly recorded (Charlesworth

1924; Dury 1964; Colhoun 1972; Ballantyne et al.

2007; Smith and Knight 2011). Eroded bedrock

features (whalebacks and rock drumlins) are also

observed on the Atlantic shelf, immediately to the

west of Aran Island (O Cofaigh et al. 2012, their

fig. 6b). No sediments are present on the seismic data

in this offshore area, and are uncommonly observed

on land. The relative absence of constructional ice-

marginal forms in west Donegal, compared to the

north coast (Stephens and Synge 1965; McCabe and

Clark 2003), has meant that processes and patterns

of ice retreat are poorly known, but large and nested

arcuate moraines are interpreted from the offshore

data, however (O Cofaigh et al. 2012). A retreat

moraine composed of boulders at Bloody Foreland,

north of this area, was dated by the 10Be cosmogenic

exposure method to 18�20 kyr BP (Clark et al. 2009).

There have been few detailed (site-scale) investiga-

tions of subglacial and ice-marginal processes in west

Donegal. From the Loughros Beg estuary, Knight

(2009, 2011) described the formation of subglacial

meltwater scours and rock-cored drumlins. From

coastal sections around Bloody Foreland, McCabe

(2005) sketched a late Midlandian stratigraphy

comprising a raised shore platform and beach,

overlain by a diamicton of uncertain origin, and

Fig. 2*Maps of (A) Aran Island and (B) Cruit Island showing the distribution of glacigenic landforms mapped by remote sensing andfield observation, and the locations of sites A�F named in the text. Contours are given in metres. Areas of ‘drift’ on Aran Island aregiven after the Geological Survey of Ireland map sheet 9 (dated 1889), although there is little support in the field for this. Hummockytopography was identified as disordered and nonaligned mounds of sediment (a few metres in height) observed in the field, andexcluding sand dune ridges.

Knight*Last glaciation of Aran Island and Cruit Island, Co. Donegal 51

overlain in turn by gravels. Such a stratigraphyappears different to that recorded in south Donegal

where two ‘head’ deposits are overlain by a glacial

diamicton (Colhoun 1973). These local stratigraphic

differences may suggest activity of a different ice

sheet sector in combination with localised sediment

sources. Despite these regional and site-scale studies,

specific mention of west Donegal islands is limited to

the description by Charlesworth (1924) of graniteerratics across all bedrock surfaces of Aran Island.

‘Banking of drift’ was also identified on south and

south-eastern slopes of Aran Island (p. 213),

although these ‘banks’ were not described in detail.

In this study, the glacial geomorphology of Aran

Island and Cruit Island was mapped from National

Coastline Survey of Ireland air photos (at a variable

scale but with a pixel resolution of � 30m), whichalso covered interior parts of the islands, and in the

field. Where available, sediment exposures were

examined for their lithological properties and sedi-

mentary structures.

Description and interpretation of glacigenic evidence

from Aran Island and Cruit Island

Geomorphological evidence

Glacial erosional features dominate across the

islands (Fig. 2). The most common property is

that of areal glacial scouring, which leaves bedrock

surfaces bare and smoothed and with moderate

local relief (a few metres in many cases). Striae

patterns are reported elsewhere (Smith and Knight

2011); this study focuses on mesoscale erosional

forms. These mesoscale forms (sensu Glasser and

Warren 1990), including whalebacks and roches

mountonnees, are relatively uncommon given that

this is a landscape dominated by erosional forms,

but mesoscale forms are best developed where the

bedrock surface slopes in a down-ice direction. This

is most clearly seen on Aran Island (site A on Fig. 2)

where hairpin-shaped, elongate and curved

symmetric hills, whose forms are 400�500m long

and 40�50m wide, are developed in quartzite bed-

rock. These landforms, which are interpreted as

flutes, have their highest points located on the up-ice

(east) side of the landform and are slightly curved in

plan view (Fig. 3). Flutes of a similar shape and

length:width ratio have been described from

Alberta, Canada, where they were interpreted as a

product of subglacial outburst floods (Shaw et al.

2000). Similar landforms, termed megagrooves,

have been identified in north-west Scotland where

they are curved, developed in bedrock, and are

500�1500m long and 20�30m wide (Bradwell 2005).

Whalebacks (4�20m long, B7m high) are sym-

metrical in long profile and have smoothed,

Fig. 3*Satellite image view of flutes on Aran Island (site A on Fig. 2). Flute margins are dotted for clarity (source: Google Earthimage, dated 18 April 2009).


low-relief surfaces. Whalebacks are best developed

on relatively low-relief rock surfaces on some of the

smaller west Donegal islands (Fig. 4A). On the east

side of Cruit Island, joint-defined granite bosses

(50�90m across) may have facilitated the develop-

ment of more rounded rather than elongate

erosional forms (e.g. Hall and Migon 2010; Olvmo

and Johansson 2002). Here, enhanced weathering

along granite joints has left a checkerboard pattern

of rounded residual hills separated by valleys or

depressions (a few metres in height) that are free of

sediment either on valley sides or valley floors, and

often intersect at right angles (e.g. Ehlen 1999).

In this setting, there is therefore a strong geologic

control on whaleback elongation. This is consistent

with studies elsewhere, such as by Olvmo and

Johansson (2002) and Evans (1996) who reported

that erosional forms on different bedrock types have

different shapes and elongation ratios.

Roches mountonnees are relatively uncommon

across Aran and Cruit Islands, because of the

absence of plucked faces on the leesides of bedrock

protrusions. This may reflect a geological control on

the development of leeside cavities, which takes

place preferentially on descending slopes and on

fractured bedrock (Sugden et al. 1992) and so is not

favoured on granite bedrock with higher rock mass

strength and fewer fractures. Large-scale erosional

forms with plucked faces are present on Aran Island

(site B on Fig. 2). These features (50�150m across

and with faces 6�14 m high) are much larger than

roches mountonnees elsewhere in the region

(e.g. Knight 2009) and show plucking on their

northerly sides, which is consistent with the direc-

tion of adjacent striae. The variable morphology

and range of scales of bedrock erosional forms

reflect a combination of topographic and geologic

control, discussed below. Meltwater channels that

are present on the eastern side of Cruit Island have

given rise to oversteepened slopes on the sides of

small valleys. In upland areas of Aran Island, cross-

slope marginal meltwater channels aligned east-west

are more clearly distinguished (site C on Fig. 2).

These are 100�200m long and up to 10m wide, and

crosscut bedrock structures.

There is little evidence for glacial deposition

across any of the west Donegal islands, including

those described here. Erratic boulders are present

across the region, but have a particularly high

concentration on parts of The Rosses and on the

southern part of Aran Island. As granites form

much of the bedrock across the region (including

The Rosses), erratic carriage is not a useful measure

of ice flow direction, although the regional distribu-tion of granite boulders has been used in this

context (Charlesworth 1924). Granite boulders

from the mainland are also found on quartzite

bedrock on the south coast of Aran Island. There is

limited carriage of quartzite boulders sourced from

this area onto granite elsewhere on Aran Island.

Where exposed, the quartzite bedrock surface is

extensively fractured and there is little evidence forglacigenic shunting or rafting. Granite boulders

appear limited to the area south of the highest

summits on Aran Island (Fig. 2). At Bellachreesh

Bay on the north coast of Aran Island, a boulder

beach is comprised of clasts almost entirely derived

from the underlying quartzite, and erratic lithologies

are rare (Fig. 4B). At Pollawaddy harbour,

deformed and fractured quartzite bedrock thatforms a coarse breccia unit (B4m thick) is present

on the steeper northern side of the harbour, whereas

non-local sediment deposition has taken place on

the south side (described below). On the south-west

coast of Aran Island a marginal moraine can be

identified (site D on Fig. 2). This feature runs

approximately slope-parallel for around 600m, and

has a maximum thickness of 3m, declining inelevation and extent to the west. The moraine has

a bench-like morphology, with a flat upper surface

that onlaps the bedrock slope behind, and a steeper

(but not ice-contact) south-facing frontal slope.

Sediments within this moraine are described below.

Sedimentary evidence

At site D located above granite but very close to

quartzite outcrops (around 10m away), small

sections show massive and poorly sorted gravels

with an equal mixture of granite and quartzite

clasts, including weathered granite corestones

(40cm diameter). No sedimentary structures are

observed and glacial diamicton is absent. At siteE, in the centre of Aran Island and flanking one of

the highest summits at Cluidaniller, a section in a

disused quarry (4m thick) shows large, isolated

granite corestones (B1m diameter) surrounded by

a breccia of weathered granite fragments and

angular quartzite clasts. Sediment sorting is poor

and no sedimentary structures are observed. The

same massive and poorly sorted sediments(6m thick) are also found locally on the south side

of Pollawaddy harbour where they onlap quartzite

bedrock. The bedrock surface is not exposed

here. At Gobdoo headland on Aran Island, the

same sediment unit (1m thick) overlies an undulat-

ing and sharply demarcated granite bedrock surface


Fig. 4*(A) View of mesoscale whaleback forms developed on granite on Inishcoo island. (B) View of the present boulder beach atBellachreesh Bay (Aran Island) showing the dominance of local quartzite clasts and absence of erratics. (C�F) Photos of site F onCruit Island, trowel for scale is 28cm long. (C) View of the transition between granitic breccia (below) and Holocene dune sand (top ofexposure). Note the vague planar stratification and granule lens (level of the trowel). (D) View of the sharply eroded granite bedrocksurface overlain by angular granite clasts and strongly amalgamated diamicton beds. (E) Fractured and displaced angular granite clastwithin the granitic breccia. (F) A clast cluster (level of the trowel) overlying gravels. The clast cluster contains a higher proportion oferratic clasts and clasts are touching, sometimes imbricated.


which shows enhanced weathering along structural

weaknesses. Rounded corestones are common with-

in the sediments, but bedrock fragments that have

been periglacially fractured and erratic materials

indicative of glacial transport are absent.

Sediments are exposed within embayments on

the western side of Cruit Island. Along this coast

(e.g. at site F on Fig. 2) a single granite-rich unit is

present below Holocene dune sand from which it is

separated by a gradational contact (Fig. 4C). The

sediments are texturally variable and include a

diamictic breccia (sensu Knight 1999, 2005) com-

posed of local bedrock fragments, matrix-dominant

diamicton, granule lenses and silt stringers. These

different facies have high lateral and vertical varia-

bility but generally sediments lower in the profile

show amalgamated and welded beds and appear

overconsolidated, whereas those higher in the pro-

file exhibit greater interbedding. The sediments

(B2m thickness) overlie a granite rock surface which

is highly undulatory with local relief of up to 1m.

The bedrock surface varies from sharply planar

erosional to showing displaced rounded granite

boulders (Fig. 4D). Sediments above this surface

are generally massive and poorly sorted with

some planar stratification observed, related to

variations in clast concentration. Isolated pods of

massive granules are also present in places (Fig. 4C).Throughout the sediments at site F, local granite

dominates the clast population (�95%). Granite

clasts occur across a wide size range but make up all

of the largest boulders (B30cm diameter) that are

present in the section. Granite clasts range from

angular to subrounded, suggesting local and more

distant source areas, respectively. More angular

clasts are usually located at the base of the section

with more rounded clasts higher up. Some angular

clasts show evidence of syndepositional clast-clast

contact and brecciation (Fig. 4E), but matrix

deformation and faulting are not observed.

Occasionally non-imbricated clasts form chaotic

clusters that are variably clast- to matrix-supported

(Fig. 4F). The clasts are of similar size and are

usually touching, occasionally imbricated. These

clusters also have a higher erratic content (5�8%)

than surrounding sediments, and include schists

derived from the south around Gweebarra Bay

(not from The Rosses to the south-east). Through-

out the sedimentary section, the matrix portion is

dominant (80 to �90%) and is composed exclu-

sively of mineral grains derived from weathered

granite, including more friable feldspars. These

mineral grains have a ‘dirty’ appearance, indicating

the presence of clay forming cutans over grainsurfaces. The clays may be a product of granite

weathering (e.g. Eppes and Griffing 2010).

Discussion

Geomorphological and sedimentary evidence from

Aran Island and Cruit Island shows that glacial

erosional processes were dominant during the Mid-

landian glaciation, forming striae, flutes developed

in bedrock, whalebacks and roches mountonnees,

and there is very limited evidence for glacialdeposition. The amount of glacial erosion is very

difficult to evaluate: ice may have merely stripped

away any pre-existing weathered debris to reveal an

unweathered bedrock surface with very minimal net

erosion. In Minnesota, smooth crystalline bedrock

surfaces were formed solely by the stripping away of

a weathered mantle, and did not require extensive

subglacial scouring (Patterson and Boerboom1999). In Scotland, granite tors were affected by a

continuum of glacial modification, from preserva-

tion beneath cold-based ice to selective block

removal (Hall and Phillips 2006). The morphology

of granite tors or exposed granite hilltops can

therefore indicate the nature of overlying ice cover

including its thermal regime. Ballantyne et al. (2007)

used this line of evidence to help identify summits inDonegal that had escaped glacial erosion.

The role of geology

The properties of the underlying granite bedrock

had a critical control on the range of landforms and

sediments produced during the last glacial cycle in

west Donegal. Pre-glacial weathering by a range of

both warm- and cold-climate processes during the

Tertiary and/or Quaternary was important in loose

sediment generation. This included (1) granular

fragments (saprolite) that are seen in situ in someplaces in Donegal and elsewhere in Ireland mantling

bedrock surfaces (Coxon 2005); (2) unweathered

bedrock boulders (corestones) surrounded by an

aureole of saprolite, which may have been the

primary mechanism of generating suitable boulders

for later carriage across Donegal; and (3) silts and

clays produced by differential chemical weathering

of feldspars (grus) (Eppes and Griffing 2010),which is seen in section at Cruit Island. The

general absence of glacigenic sediments across west

Donegal, and the presence of widespread glaciated

rock surfaces, suggests either that surficial pre-

glacial sediments were absent, or that any sediments

were glacially stripped, revealing the underlying


bedrock (e.g. Olvmo and Johansson 2002). The roleof pre-glacial weathering in landscape denudation of

limestones has been demonstrated elsewhere in

Ireland (Coxon 2005) but there is little information

on weathering products from granite substrates in

Donegal. One clue, however, may come from

geochemical analysis of surface sediments found

above the limits of glacial erosion on mountain tops,

including the diagnostic secondary mineral gibbsitewhich is formed by silicate weathering (Paasche

et al. 2006). The presence of this mineral may

indicate the extent to which weathered products

have remained in situ rather than having been

glacially stripped (Ballantyne 1998). Constructional

end moraines on the Atlantic shelf may support the

hypothesis of widespread glacial stripping in west

Donegal.

Sedimentary processes

On Aran Island and Cruit Island, sediments at

Pollawaddy Harbour, Gobdoo and site F are

restricted to bedrock hollows and do not have a

geomorphological expression. This may suggest thatthese sediments were deposited during ice advance,

and the tops of the hollows planed off, or that

sediments in these hollows have higher preservation

potential irrespective of formative process. The high

matrix content of these sediments also suggests that

they were formed by pre-glacial weathering. If this

timing of sediment deposition is correct, the erratic

content at site F suggests initial ice flow from thesouth or south-east. The relative chronology of

bedform flow sets in the region is too uncertain to

either prove or disprove this (Greenwood and Clark

2009).

The physical properties of these sediments

include: dominance of underlying granite and very

low erratic content; very poor sorting and angular,

fractured clasts; and no sedimentary or tectonicstructures indicative of ice-bed interaction, such as

shearing or overfolding. These properties are not

typical of glacial diamicton (till). Croot and Sims

(1996) described a continuum of glacial diamicton

types that reflects both transport distance from a

bedrock source and the extent to which materials

have experienced glacial comminution by shearing.

These diamicton types are: bedrock rafts whichgrade to glacitectonic breccias, to immature till,

and to mature till. The lack of evidence for

glacitectonic processes in west Donegal suggests

that the non-genetic term diamictic breccia is more

appropriate for such sediments, as has been used

elsewhere (e.g. Knight 1999, 2005). The sediments

observed at site F are mainly locally deriveddiamictic breccias, but in the middle of the section,

clasts within the clast cluster (Fig. 4F) reflect glacial

transport if not glacial deposition itself. The absence

of sedimentary structures, lack of clear stratification

and upward change in sediment type and source

suggest that subglacial deposition took place by

slumping into hollows on the bedrock surface which

were progressively infilled. The lack of sediment onsurrounding bedrock surfaces suggests that this is a

sediment-limited system, which has implications for

ice sheet dynamics.

Implications for late Midlandian ice advance and

retreat in west Donegal

Striae evidence from across Donegal, extended

across the west Donegal coast, argues for an initial

east�west ice flow followed by a south-east�north-

west ice flow (Smith and Knight 2011). However,

there is uncertainty in the timing and reach of these

phases, and similar patterns can be produced by

migration of the Donegal dome ice centre without

any major change in ice volume or extent. Theabsence of depositional subglacial signatures may

highlight the role of erosional stripping of any pre-

existing sediments, but it may also reflect the nature

of weathered granite itself. High subglacial pore

water pressures are hard to maintain in free-drain-

ing coarse sediments (Moore and Iverson 2002),

which do not favour the development of deforma-

tion tills and the formation of net depositionallandforms. It is likely, however, that hard and

impermeable granite surfaces with low fracture

density lead to higher subglacial meltwater avail-

ability, higher hydraulic pressure and fast ice flow by

ice surging or streaming (Kamb 1987).

There is some evidence for the development of

fast ice flow in west Donegal. Drumlin formation

within west Donegal embayments (Wright 1912;Knight and McCabe 1997; Knight 2011) is most

commonly associated with convergent and fast ice

flow (Briner 2007). Transport of sediment to west

Donegal marginal shelf positions (O Cofaigh et al.

2012) requires an integrated sediment conveyor

system, which works most efficiently under high

ice velocities (Swift et al. 2002). Streamlined bed-

forms indicative of fast ice flow are also observed offthe west Donegal coast (O Cofaigh et al. 2012). The

absence of deglacial sediments and end moraines

across west Donegal suggests that ice retreat from a

maximal shelf position was rapid and possibly took

place by in situ stagnation or collapse rather than by

retreat at an active ice margin (e.g. Dowdeswell et al.


2008). This is consistent with likely styles of

deglaciation elsewhere in Ireland (McCabe et al.

1998). Erratic boulders were deposited during ice

retreat phases. Their seemingly random distribution

(Charlesworth 1924) shows they were not deposited

consistently at marginal stillstands, although their

precise source areas, travel paths, morphologies and

ages have not yet been determined. If they were

deposited from an ice margin, it suggests consider-

able ice thinning between modelled ice sheet thick-

ness over the region during the LGM (500m; Fig. 1),

and subsequent ice retreat on land. The marginal

moraine seen on Aran Island (site D on Fig. 2) was

formed when the retreating ice margin impinged

against emerging upland bedrock surfaces. Its posi-

tion implies that ice was in retreat towards the south

or south-east. Its relationship to deglaciation of

islands farther north and to the Bloody Foreland

moraine (Clark et al. 2009) is unclear. This remains

a problem for future studies.

Conclusions

Glacigenic landforms and sediments from Aran

Island and Cruit Island, supported by regional

evidence across west Donegal, illustrate the pro-

cesses and dynamics of the late Midlandian ice

sheet. Erosional landforms dominate, but the prop-

erties of the underlying granite substrate exerted a

strong control on pre-glacial weathering (generating

loose debris) and favoured sediment stripped by the

advancing ice sheet. These properties include its

weathering and erosional history. The absence of

depositional subglacial or ice-marginal signatures

(i.e. diamicton drumlins and moraines, respectively)

suggests that, following fast ice flow, ice down-

wasting and in situ disintegration was rapid. Erratic

boulders may have been let down passively onto

deglaciated surfaces at this time. The general

absence of glacial landforms and sediments in west

Donegal highlights the critical role of bedrock

geology on subglacial processes and ice sheet

dynamics. This is a poorly-understood control on

the Irish ice sheet (Greenwood and Clark 2010) but

it is significant because it may amplify or retard any

climate forcing on ice sheet dynamics. Ice sheet-scale

models of glacier-substrate interactions, similar to

those developed for the Laurentide ice sheet

(e.g. Marshall et al. 1996), could be usefully applied

to Ireland to better understand these controls on ice

sheet dynamics.

Acknowledgements

I thank Helene Burningham for field assistance and

Steve McCarron for his comments on this paper.

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JASPER KNIGHT,School of Geography, Archaeology and Environmental Studies,University of the Witwatersrand,Private Bag 3,Wits 2050,Johannesburg,South Africa.

E-mail: [email protected]


Documents

The last glaciation of Aran Island and Cruit Island, County Donegal, north-west Ireland